The NF-κB (nuclear factor κB) family is composed of homo- and heterodimers of the Re1 family of transcription factors. A key role of these transcription factors is to induce and coordinate the expression of a broad spectrum of pro-inflammatory genes including cytokines, chemokines, interferons, MHC proteins, growth factors and cell adhesion molecules (for reviews see Verma et. al., Genes Dev. 9:2723–35, 1995; Siebenlist et. al., Ann. Rev. Cell. Biol. 10:405–455, 1994; Bauerle and Henkel, Ann. Rev. Immunol., 12:141–179, 1994; Barnes and Karin, New Engl. J. Med., 336:1066–1071, 1997).
The most commonly found Re1 family dimer complex is composed of p50 NFkB and p65 Re1A (Baeuerle and Baltimore, Cell 53:211–217, 1988; Baeuerle and Baltimore, Genes (Dev. 3:1689–1698, 1989). Under resting conditions NF-κB dimers are retained in the cytoplasm by a member of the IκB family of inhibitory proteins (Beg et. al., Genes Dev., 7:2064–2070, 1993; Gilmore and Morin, Trends Genet. 9:427–433, 1993; Haskil et. al., Cell 65:1281–1289, 1991). However, upon cell activation by a variety of cytokines or other external stimuli, IκB proteins become phosphorylated on two critical serine residues (Traenckner et. al., EMBO J., 14:2876, 1995) and are then targeted for ubiquitination and proteosome-mediated degradation (Chen, Z. J. et. al., Genes and Dev: 9:1586–1597, 1995; Scherer, D. C. et. al., Proc. Natl. Acad. Sci. USA 92:11259–11263, 1996; Alkalay, I. et. al., Proc. Natl. Acad. Sci. USA 92:10599–10603, 1995). The released NF-κB is then able to translocate to the nucleus and activate gene transcription (Beg et. al., Genes Dev., 6:1899–1913, 1992).
A wide range of external stimuli have been shown to be capable of activating NF-κB (Baeuerle, P. A., and Baichwal, V. R., Adv. Immunol., 65:111–136, 1997). Although the majority of NF-κB activators result in IκB phosphorylation, it is clear that multiple pathways lead to this key event. Receptor-mediated NF-κB activation relies upon specific interactions between the receptor and adapter/signalling molecules (for example, TRADD, RIP, TRAF, MyD88) and associated kinases (IRAK, NIK) (Song et. al., Proc. Natl. Acad. Sci. USA 94:9792–9796, 1997; Natoli et. al., JBC 272:26079–26082, 1997). Environmental stresses such as UV light and γ-radiation appear to stimulate NF-κB via alternative, less defined, mechanisms.
Recent publications have partially elucidated the NF-κB activation. This work has identified three key enzymes which regulate specific IκB/NF-κB interactions: NF-κB inducing kinase (NIK) (Boldin et. al., Cell 85:803–815, 1996), IκB kinase-1 (IKK-1) (Didonato et. al., Nature 388:548,1997; Regnier at al., Cell 90:373 1997) and IκB kinase-2 (IKK-2) (Woronicz et. al., Science 278:866, 1997; Zandi et. al., Cell 91:243, 1997).
NIK appears to represent a common mediator of NF-κB signalling cascades triggered by tumour necrosis factor and interleukin-1, and is a potent inducer of IκB phosphorylation. However NIK is unable to phosphorylate IκB directly.
IKK-1 and IKK-2 are thought to lie immediately downstream of NIK and are capable of directly phosphorylating all three IκB sub-types. IKK-1 and IKK-2 are 52% identical at the amino acid level but appear to have similar substrate specificities; however, enzyme activities appear to be different: IKK-2 is several-fold more potent than IKK-1. Expression data, coupled with mutagenesis studies, suggest that IKK-1 and IKK-2 are capable of forming homo- and heterodimers through their C-terminal leucine zipper motifs, with the heterodimeric form being preferred (Mercurio et. al., Mol. Cell Biol., 19:1526, 1999; Zandi et. al., Science; 281:1360, 1998; Lee et. al, Proc. Natl. Acad. Sci. USA 95:9319, 1998).
NIK, IKK-1 and IKK-2 are all serine/threonine kinases. Recent data has shown that tyrosine kinases also play a role in regulating the activation of NF-κB. A number of groups have shown that TNF-α induced NF-κB activation can be regulated by protein tyrosine phosphatases (PTPs) and tyrosine kinases (Amer et. al., JBC 273:29417–29423, 1998; Hu et. al., JBC 273:33561–33565, 1998; Kaekawa et. al., Biochem. J. 337:179–184, 1999; Singh et. al., JBC 271 31049–31054, 1996). The mechanism of action of these enzymes appears to be in regulating the phosphorylation status of IκB. For example, PTP1B and an unidentified tyrosine kinase appear to directly control the phosphorylation of a lysine residue (K42) on IκB-α, which in turn has a critical influence on the accessibility of the adjacent serine residues as targets for phosphorylation by IKK.
Several groups have shown that IKK-1 and IKK-2 form part of a ‘signalosome’ structure in association with additional proteins including IKAP (Cohen et. al., Nature 395:292–296, 1998; Rothwarf et. al., Nature 395:297–300, 1998), MEKK-1, putative MAP kinase phosphatase (Lee et. al., Proc. Natl. Acad. Sci. USA 95:9319–9324, 1998), as well as NIK and IκB. Data is now emerging to suggest that although both IKK-1 and IKK-2 associate with NIK, they are differentially activated, and therefore might represent an important integration point for the spectrum of signals that activate NF-κB. Importantly, MEKK-1 (one of the components of the putative signalosome and a target for UV light, LPS induced signalling molecules and small GTPases) has been found to activate IKK-2 but not IKK-1. Similarly, NIK phosphorylation of IKK-1 results in a dramatic increase in IKK-1 activity but only a small effect on IKK-2 (for review, see Mercurio, F., and Manning, A. M., Current Opinion in Cell Biology, 11:226–232, 1999).
Inhibition of NF-κB activation is likely to be of broad utility in the treatment of inflammatory disease.
There is accumulating evidence that NF-κB signalling plays a significant role in the development of cancer and metastasis. Abnormal expression of c-Re1, NF-κB2 or IκBα have been described in a number of tumour types and tumour cell lines, and there is now data to show that constitutive NF-κB signalling via IKK2 takes place in a wide range of tumour cell lines. This activity has been linked to various upstream defects in growth factor signalling such as the establishment of autocrine loops, or the presence of oncogene products e.g. Ras, AKT, Her2, which are involved in the activation of the IKK complex. Constitutive NF-κB activity is believed to contribute to oncogenesis through activation of a range of anti-apoptotic genes e.g. A1/Bfi-1, IEX-1, XIAP, leading to the suppression of cell death pathways, and transcriptional upregulation of cyclin D1 which promotes cell growth. Other data indicate that this pathway is also likely to be involved in the regulation of cell adhesion and cell surface proteases. This suggests a possible additional role for NF-κB activity in the development of metastasis. Evidence confirming the involvement of NF-κB activity in oncogenesis includes the inhibition of tumour cell growth in vitro and in vivo on expression of a modified form of IκBα (super-repressor IκBα).
In addition to the constitutive NF-κB signalling observed in many tumour types, it has been reported that NF-κB is also activated in response to certain types of chemotherapy. Inhibition of NF-κB activation through expression of the super-repressor form of IκBα in parallel with chemotherapy treatment has been shown to enhance the antitumour effect of the chemotherapy in xenograft models. NF-κB activity is therefore also implicated in inducible chemoresistance.